Air emissions

Limiting and reducing air pollution in a modern industrial society is an ambitious and complex undertaking. The Clean Air Act, in its progressive iterations over several decades, is the primary tool for the U.S. Environmental Protection Agency to carry out this task.

The 1970 Clean Air Act introduced binding national standards for air quality. Congress enacted major revisions to the law in 1977 and 1990.

In brief, the law:

• sets air quality standards for the nation as a whole, focusing on limits for six pollutants.
• includes a separate regulatory regime for hazardous pollutants.
• allows EPA to establish emission limits for new and modified stationary sources of pollution — primarily power plants and industrial facilities, although also including wood stoves.
• requires strict limits on certain pollutants emitted by vehicles.
• requires plans by states to clean up dirty air and protect clean air.
• requires designation of dirty- and clean-air areas, which are then subject to different regulatory requirements.
• includes a program to prevent deterioration in areas with cleaner air than required by national standards.
• includes a program to protect visibility in pristine areas such as national parks and wilderness areas.

In 2007, the U.S. Supreme Court ruled that EPA must add carbon dioxide to the list of pollutants that require emission standards — giving the agency the authority to address climate change.
 

The U.S. Environmental Protection Agency has documented dramatic improvements in air quality under the Clean Air Act.

For example, between 1980 and 2021, concentrations of major pollutants have been reduced:

• carbon monoxide by 87 percent.
• nitrogen dioxide by 67 percent.
• lead by 98 percent.
• ground-level ozone by 29 percent.
• sulfur dioxide by 94 percent.

The agency notes that 102 million Americans live in areas with pollution levels exceeding national standards for at least one of the major pollutants.
 

While the effort to put in place regulations for carbon dioxide emissions remains a work in progress, the United States has shown modest reductions in overall greenhouse gas emissions between 1990 and 2020.

In that time period, overall greenhouse gas emissions have declined by 7.3 percent, according to the U.S. Environmental Protection Agency:

• carbon dioxide emissions are down 7.9 percent.
• methane emissions are down 16.7 percent.
• nitrous oxide emissions are down 5.4 percent.
• fluorinated gas emissions are up 89.8 percent.
   

Carbon dioxide (CO2) is a colorless, odorless, tasteless gas about 1.5 times as dense as air, released by plant and animal respiration and consumed by photosynthesis. It also is a product of combustion of carbon-containing materials, such as fossil fuels. Carbon dioxide is the primary heat-trapping greenhouse gas.

Methane is the chief constituent of natural gas, produced by decaying plant material or by coal gasification processes. Methane is a powerful greenhouse gas.